1. Field of the Invention:
This invention relates to a process for the purification of hydrogen gas with a cyclical charging and discharging of metal hydride reservoirs as described herein, and to an apparatus for the execution of this process.
2. Description of the Prior Art:
The prior art describes hydride reservoirs for the purification of contaminated hydrogen gas. Use is made of the fact that a reservoir mass consisting of a hydride-forming metal, when charged with hydrogen, absorbs the hydrogen portion and binds it in the metal matrix, while the impurities are either chemisorbed on the surface or remain unbonded. A relatively pure gas is then obtained by releasing the hydrogen when the reservoir is discharged.
The concentration of the hydrogen in the reservoir mass is a function of the temperature and the gas pressure. When the hydride reservoir is charged, heat is released (enthalpy of formation). Conversely, when the reservoir is discharged, heat is required to drive the hydrogen out of the hydride material. Conventional systems are used to heat and cool the reservoir. That means, for example, the use of naturally-occurring surface or ground water for cooling, and the use of fossil fuels for heating. The energy consumed is on the order of 0.5 kWh/m3 of hydrogen, and therefore represents a significant operating cost factor.
U.S. Pat. No. 3,516,263 relates to such a process for purifying hydrogen through absorbtion and desorption by employing a reversible metal hydride forming reaction.
U.S. Pat. No. 4,444,727 relates to a hydrogen purification system which includes two metal hydride reservoirs working in alternation. The two hydride reservoirs are each equipped with a heat exchanger, and are coupled to one another for the purpose of mutual heat exchange. In other words, the heat released during the charging of one hydride reservoir with raw hydrogen gas is conducted via the two heat exchangers and the system of lines connecting them to the other hydride reservoir, which is being discharged and therefore requires a corresponding amount of heat to release the pure hydrogen gas. This solution substantially reduces the energy consumption for hydrogen purification.
This purification installation, however, is unsatisfactory from several points of view. One particular disadvantage is that the reservoir mass of both hydride reservoirs becomes more contaminated with every charge/discharge cycle by chemisorption of certain impurities in the raw hydrogen gas, so that its purification capacity decreases steadily over time, until it becomes completely unusable. After a certain period of operation, therefore, it becomes necessary to replace both hydride reservoirs.
Another disadvantage is that, before the first charging of the hydride reservoir with raw hydrogen, the reservoir volume must be evacuated and thereby freed of impurities. After any discharge, however, impurities remain which are not absorbed by the reservoir mass, and are contained in the so-called "pure" gas discharged. This is due to the fact that no precautions have been taken to otherwise remove the contaminated top gas at the beginning of a discharge cycle, and only after the hydride reservoir has been flushed with released hydrogen gas is genuinely high-grade pure hydrogen gas actually discharged to the hydrogen consumers.
For this reason, the quality of the pure gas fluctuates during a cycle. If one wishes to eliminate this shortcoming and provide corresponding devices to remove the contaminated top gas, then another shortcoming becomes apparent: During the release of the top gas, both hydride reservoirs may have to be discharged simultaneously, so that heat must be supplied to both of them simultaneously. But at least in this phase, that destroys the basic operating principle of the mutual heat exchange proposed by U.S. Pat. No. 4,444,727, with the result that, at least temporarily, the continuous delivery of pure gas cannot be guaranteed. To eliminate this disadvantage, there can, for example, be a pure gas buffer reservoir or a significant oversizing of the hydride reservoir. In the latter case, only as much of the actual capacity of the hydride reservoir being discharged is used, so that in spite of the absence of heat supplied during the discharge of the top gas from the other hydride reservoir and the consequent drop in the reservoir temperature, sufficient quantities of hydrogen with a sufficient discharge pressure can be provided. Both solutions result in a significant increase in the cost of the purification installation. This is also true for a special variant of a similar purification installation shown in U.S. Pat. No. 4,444,727, in which the heat exchange between the two hydride reservoirs takes place by means of an intermediate heatsink, which, to a certain extent, can be considered a thermal buffer. Such an installation entails additional expense, not only for the heatsink, the connecting lines and the circulation pumps, but also for service and maintenance of these pieces of equipment.
The above-mentioned U.S. Pat. Nos. 4,444,727 and 3,516, 263 are hereby incorporated by reference, as if the entire contents thereof were expressly set forth herein.